1. Field of the Disclosure
The present disclosure relates to an improved process for making titanium dioxide, and in particular to a process for gas phase oxidation that results in titanium dioxide particles having improved particle size and distribution.
2. Background of the Disclosure
Titanium dioxide pigments have a variety of applications, including coatings, paints, plastics, paper, etc. Light scattering is one of the important properties of these pigments and it is very dependant on particle size and on particle size distribution.
In producing pigmentary titanium dioxide (TiO2), a titanium tetrahalide such as titanium tetrachloride (TiCl4) is reacted in the vapor phase with an oxygen-containing gas in a reactor at a temperature in the range of about 900° to 1600° C. to produce a hot gaseous suspension of TiO2 solid particulates. This oxidation reaction has a high activation energy requiring a significant amount of energy to preheat the reactants. In addition, this reaction is exothermic and results in extremely high temperatures in the reactor causing the titanium dioxide particle growth rate in the reactor to increase. The hot gaseous suspension of titanium dioxide solid particulates must then be quickly cooled below 600° C. within about 1-60 seconds following discharge of the suspension from the reactor. This cooling is accomplished in a conduit, e.g., a flue, which is externally cooled with flowing water so that undesired TiO2 particle size growth is prevented and particle agglomeration is minimized. The titanium dioxide formed can then be subjected to wet treatment, filtration, and drying before the particles are subjected to micronization processes to provide a finished pigment product.
The particle size of the TiO2 pigment can be measured in terms of carbon black undertone (CBU). Pigments containing smaller sized particles have a relatively high CBU, and finished products (e.g., paints, plastics, etc.) containing such pigments tend to have a bluish tint. Pigments with larger sized particles have a relatively low CBU and finished products containing such pigments tend to have a more yellowish tint. The particle agglomeration of the pigment is typically measured in terms of its particle size distribution (coarse tail). Pigments, wherein a low weight percentage of the particles (e.g., less than 30%) have a particle diameter size greater than 0.6 microns, tend to have low particle agglomeration and finished products made with such pigments tend to have high gloss. Pigments, wherein a high weight percentage of the particles have a particle diameter size greater than 0.6 microns, tend to have greater particle agglomeration and finished products made with such pigments tend to have less gloss.
A need exists for an improved energy efficient process that results in particles having an improved size and size distribution.
A process for controlling the formation of titanium dioxide particles in a multi-stage vapor-phase oxidation reactor has been proposed wherein the fraction of gaseous titanium tetrachloride converted to titanium dioxide in a first stage is limited and the remaining fraction of the gaseous titanium tetrachloride is introduced in a subsequent stage for reaction with unreacted oxygen of the first stage. While the process is said to facilitate titanium dioxide particle size control and particle size distribution, process inefficiencies associated with obtaining and maintaining the titanium tetrachloride feed in the gas phase, employing separate facilities for converting the liquid feed into gas or employing one such facility but then separating the gaseous feed and controlling the proportion of gaseous titanium tetrachloride introduced to the different stages are significant drawbacks.